The present invention relates to a current detection circuit, a power supply apparatus using the current detection circuit, a power supply system using the current detection circuit, and an electronic apparatus using the current detection circuit.
In recent years, switching power supplies are most commonly used as the power supplies for various electronic apparatuses. In general, switching power supplies contain a current detection circuit that converts the output currents of these switching power supplies into current detection voltages. These current detection voltages are used for an overcurrent protection circuit to protect the particular power supply from a load overcurrent, and for a parallel operation circuit to operate multiple such power supplies in parallel. The overcurrent protection circuit protects the appropriate power supply by conducting a power supply shutdown process and other processes if the occurrence of an overcurrent in the power supply due to an unusual event such as the abnormality of a load causes the current detection voltage to exceed a preset threshold voltage value. During parallel operation of multiple power supplies with their outputs connected in parallel to one another, the parallel operation circuit accurately recognizes the output currents of the power supplies, including the plus/minus direction of the particular current, and conducts control for an equal output current between the power supplies by, for example, adjusting the voltage settings of each power supply according to recognition results. The current detection voltage of the current detection circuit is needed to recognize the output currents of each power supply. Specific examples of the known methods employed for parallel operation circuits include the droop method, the average current method, the maximum current method, etc. (one such method is described in, for example, Japanese Patent Laid-Open No. Hei 9-93929).
A known circuit method of detecting the output currents of the power supplies required for a parallel operation circuit is by inserting a current detection shunt resistor in series between the output terminals of the power supplies and detecting the voltage developed across this resistor (refer to, for example, Japanese Patent No. 2990742). However, this method has the disadvantage that the shunt resistor generates a great deal of heat. Known method as one circuit method for solving this problem is by supplying to the shunt resistor a diode-rectified primary current of the transformer used for a switching power supply, and detecting the voltage developed across the resistor (refer to, for example, Japanese Patent Laid-Open No. 2001-103741). In this method, the primary current of the transformer is generally from as small as several fractions of a secondary current, to several tenths of fractions thereof. This makes it possible to reduce the value of the current flowing into the shunt resistor and hence to reduce any loss caused by the shunt resistor.
An example of a conventional switching power supply circuit composition which uses a current detection circuit is shown in FIG. 5. In the switching power supply circuit composition shown in FIG. 5, a DC power supply 51 is connected to an inverter circuit 52, the output of the inverter circuit 52 is connected to the primary winding 53a of a transformer 53, and the secondary winding 53b of the transformer 53 is connected to a rectifier circuit 54. Also, the output of the rectifier circuit 54 is connected to a smoothing circuit 55 and the output of the smoothing circuit 55 is connected to power supply outputs 55c, 55d. In addition, the primary winding 56a of a current transformer 56 is connected at a junction between the inverter circuit 52 and the primary winding 53a, the secondary winding 56b of the current transformer 56 is connected to a diode bridge 57, the output of the diode bridge 57 is connected to a resistor R51, both ends of the resistor R51 are connected to outputs 59a, 59b of a current detection circuit 58, and the outputs 59a, 59b of the current detection circuit 58 are connected to an overcurrent protection circuit 61. The overcurrent protection circuit 61 protects the appropriate power supply by conducting a power supply shutdown process and other processes if the occurrence of an overcurrent in the power supply due to an unusual event such as the abnormality of a load causes a current detection voltage V55 to exceed a preset threshold voltage value. A control circuit 62 controls the various internal components of the switching power supply circuit.
The inverter circuit 52 is of the full-bridge composition with four MOS-FETs, Q51 to Q54, and in this composition, a voltage V51 that has been input from the DC power supply 51 is switched by on/off operation of the MOS-FETs Q51 to Q54 to thereby generate an AC voltage, and AC voltage V52 is input to the primary winding 53a of the transformer 53. A capacitor 52a is a bypass capacitor. The transformer 53 steps down the input AC voltage according to the particular turns ratio of the transformer 53, and then outputs the stepped-down AC voltage to the secondary winding 53b of the transformer 53. The rectifier circuit 54 rectifies the stepped-down AC voltage output of the secondary winding 53b synchronously with MOS-FET Q55/Q56 switching, thus generating a unidirectional pulse voltage. After the pulse voltage has been rectified by the rectifying circuit 54, the smoothing circuit 55 converts the rectified pulse voltage into a DC voltage by smoothing with a low-pass filter composed of a coil 55a and a capacitor 55b, and outputs the DC voltage to the power supply outputs 55c, 55d. 
FIG. 8 shows a timing chart of the major circuit operation waveforms developed when power supply output current I52 of the switching power supply circuit composition in FIG. 5 flows in a plus direction. The circuits operate with a duration from t1 to t5 as one period. The MOS-FET Q51 to Q54 of the inverter circuit 52 are controlled in terms of phase shift. During the phase shift control, the Q51 to the Q54 are each driven at a turn-on time duty ratio of 0.5, turn-on of the Q51 alternates with that of the Q52, and turn-on of the Q53 alternates with that of the Q54. The turn-on timing of the Q53 and the turn-off timing of the Q54 are delayed by a fixed time (a time period from t1 to t2) behind the turn-on timing of the Q51 and the turn-off timing of the Q52, respectively, and the turn-off timing of the Q53 and the turn-on timing of the Q54 are delayed by a fixed time (a time period from t3 to t4) behind the turn-off timing of the Q51 and the turn-on timing of the Q52, respectively. Also, the MOS-FET Q55 of the rectifier circuit 54 operates synchronously with the turn-on/turn-off timing of the Q51, and the MOS-FET Q56 of the rectifier circuit 54 operates synchronously with the turn-on/turn-off timing of the Q52.
During the time period from t1 to t2, since the Q51 and the Q54 are turned on and the Q52 and the Q53 are turned off, the voltage V52 on the primary winding 53a of the transformer 53 becomes a plus voltage. Also, since the Q55 is turned on and the Q56 is turned off, the current to the transformer secondary winding 53b flows from the power supply output 55d to the secondary winding 53b, the MOS-FET Q55, the coil 55a, and the power supply output 55c, in that order, and current I51 to the primary winding 53a of the transformer 53 flows in a plus direction.
During the time period from t2 to t3, since the Q51 and the Q53 are turned on and the Q52 and the Q54 are turned off, both ends of the primary winding 53a are shunted by the inverter circuit 52 and thus the primary winding voltage V52 of the transformer 53 becomes zero. Also, since the Q55 is turned on and the Q56 is turned off, the current to the transformer secondary winding 53b flows from the power supply output 55d to the secondary winding 53b, the MOS-FET Q55, the coil 55a, and the power supply output 55c, in that order, and the current I51 to the primary winding 53a of the transformer 53 flows in a plus direction.
During a period from t3 to t4, since the Q52 and the Q53 are turned on and the Q51 and the Q54 are turned off, the voltage V52 on the primary winding 53a of the transformer 53 becomes a minus voltage. Also, since the Q56 is turned on and the Q55 is turned off, the current to the transformer secondary winding 53b flows from the power supply output 55d to the secondary winding 53b, the MOS-FET Q56, the coil 55a, and the power supply output 55c, in that order, and current I51 to the primary winding 53a of the transformer 53 flows in a minus direction.
During a period from t4 to t5, since the Q52 and the Q54 are turned on and the Q51 and the Q53 are turned off, both ends of the primary winding 53a are shunted by the inverter circuit 52 and thus the primary winding voltage V52 of the transformer 53 becomes zero. Also, since the Q56 is turned on and the Q55 is turned off, the current to the transformer secondary winding 53b flows from the power supply output 55d to the secondary winding 53b, the MOS-FET Q56, the coil 55a, and the power supply output 55c, in that order, and the current I51 to the primary winding 53a of the transformer 53 flows in a minus direction.
Controlling the amounts of phase shift that are equivalent to the time periods from t1 to t2 and from t3 to t4 makes it possible to control the pulse width of the voltage V52 on the primary winding 53a of the transformer 53. In addition, stepping down the voltage V52 at the turns ratio of the transformer 53, then activating the rectifier circuit 54 to rectify the stepped-down voltage, and smoothing the rectified voltage into DC voltage form with the smoothing circuit 55 makes it possible to control a power supply output voltage V53 by controlling the amounts of phase shift.
An amplitude of the current I51 is determined by the power supply output current I52 and the turns ratio of the transformer 53. For example, if the power supply output current I52 is 300 A and the transformer 53 has 30 T (turns) on the primary winding 53a and 1 T (turn) on the secondary winding 53b, the amplitude of the current I51 is expressed as 300 A/(30 T/1 T)=10 A. Thus, it can be seen that the amplitude of the current I51 is proportional to the power supply output current I52 and that an associated coefficient of proportionality is determined by the turns ratio of the transformer 53. In addition, since the primary winding 53a of the transformer 53 is shunted by the inverter circuit 52 during the periods from t2 to t3 and from t4 to t5, the current I51 of the primary winding 53a does not become zero and the current determined by the power supply output current I52 and the turns ratio of the transformer 53 continues to flow. Hence, even when the above amounts of phase shift change and the power supply output voltage V53 changes, if the power supply output current I52 is constant, a waveform of the current I51 is constant.
Transforming the current I51 with a current transformer 56, then rectifying this current into a direct current I54 using a diode bridge 57, and conducting this direct current into the resistor R51 makes it possible to obtain a current detection voltage V55 proportional to the power supply output current I52. Suppose, for example, that the power supply output current I52 is 300 A, that the transformer 53 has 30 T (turns) on the primary winding 53a and 1 T (turn) on the secondary winding 53b, that the current transformer 56 has 1 T (turn) on its primary winding 56a and 100 T (turns) on its secondary winding 56b, and that the resistor R51 has a value of 10 Ù. In this case, a current detection voltage V55 of 300 A/(30 T/1 T)/(100 T/1 T)×10 Ù=1 V can be obtained and a current detection sensitivity of the current detection circuit becomes 1 V/300 A.